In the field of nondestructive inspection, piezoelectric ultrasonic and electromagnetic ultrasonic are commonly used to detect defects such as delamination, shrinkage, holes, inclusions, bubble, inside crack of metal material such as steel plates, so as to meet growing market demands for high quality steel such as ship plate steel and high pressure vessel steel. Piezoelectric ultrasonic inspection devices are gradually replaced by electromagnetic ultrasonic inspection devices because the piezoelectric ultrasonic inspection devices have following problems in operation such as a narrow operating temperature range for detecting steel plate, consuming a large amount of coupling pure water or other coupling agent, susceptible to interference, resulting in a misjudgment. The electromagnetic ultrasonic inspection devices do not use coupling agent, and do not need to contact an object to be detected, and can directly detect a moving object or an object having a rough surface, rust and a paint layer, without interference, and have high detection accuracy.
RU2298180 discloses using a plurality of electromagnetic ultrasonic transducers to detect defects of steel plates on-line. Each electromagnetic ultrasonic transducer includes a separate cylinder, which moves vertically, to drive the electromagnetic ultrasonic transducer to move vertically. With such a structure, the electromagnetic ultrasonic transducers can be selected to operate or not operate, so as to satisfy different sizes of steel plates to be detected or different detection surfaces. WO2007/013836A1 discloses using compressed air to pass through an electromagnetic ultrasonic transducer to form an air cushion layer between the electromagnetic ultrasonic transducer and a steel plate to be detected. When the steel plate moves upward, a distance between the electromagnetic ultrasonic transducer and the steel plate is shortened, the air cushion layer is reduced, pressure is increased, and thus the electromagnetic ultrasonic transducer is driven to automatically move upward following the steel plate. When using such a structure, at a moment when the electromagnetic ultrasonic transducer moves to an operating position from a waiting position, the electromagnetic ultrasonic transducer easily collides with the steel plate, causing wearing of the detection surface. Further, the flow of the compressed air used to form the air cushion layer may be reduced because compressed air pipeline is partially blocked or other reasons, the pressure is not enough, and the electromagnetic ultrasonic transducer is not balanced which causes the electromagnetic ultrasonic transducer to contact the steel plate and wear the detection surface. In addition, the above system requires a large air supply system, and thus has a very complex structure.